The present invention relates in general to a chemical vapor deposition apparatus and, more particularly, to a protective gas shield apparatus for use during chemical vapor deposition processing.
Chemical vapor deposition (CVD) systems are used to form a thin, uniform layer or film on a substrate such as a semiconductor wafer. During CVD processing, the substrate is exposed to one or more chemical vapors such as silane, phosphane, diborane and the like, and gaseous substances such as oxygen. The gases mix and interact with the other gases and/or the surface of the substrate to produce the desired film. The desired reactions generally occur at elevated temperatures, for example 300-550xc2x0 C., with the substrate and chamber being heated to the appropriate temperature for a selected process. In many applications including semiconductor processing, film characteristics such as purity and thickness uniformity must meet high quality standards. The substrate is positioned in a clean, isolated reaction chamber to obtain high quality films. CVD systems typically employ injectors which deliver the gaseous substances directly to the surface of the substrate. An exhaust system removes waste products such as unreacted gases and powders formed during the reaction from the reaction chamber. The reaction chamber must be periodically cleaned to remove films which are deposited on the exposed surfaces of the chamber over time, eliminating sources of particulate contamination which may become embedded in the film or degrade the film thickness uniformity.
The injection ports are typically positioned less than one inch from the surface of the substrate, for example xe2x85x9 inch to xc2xd inch. With this limited clearance between the injector and the substrate surface, the surfaces of the injector will become coated with the material produced during the reactions. To reduce the amount of build-up in this area, some CVD systems include protective shields which are positioned in front of the injectors and exhaust port. An inert gas such as nitrogen is delivered through a perforated screen carried by the shield to slow the rate at which materials accumulate on shield. The high temperatures in the CVD reaction chamber create thermal stresses in the perforated screen which may cause the screen to buckle after a period of time. The buckling disrupts the uniform flow of nitrogen through the screen as well as disturbing gas flow within the chamber, leaving portions of the screen unprotected against the accumulation of deposition materials. The ability of the screen to deliver nitrogen to the reaction chamber is further reduced as the screen becomes coated with deposition materials, requiring removal and cleaning or replacement of the shield.
A number of protective shield configurations have been disclosed in U.S. Pat. Nos. 5,849,088 and 5,944,900, the entire disclosures of which are hereby incorporated by reference herein. The descriptions provide specific design details that enhance the performance of the shields. In particular, useful life of the shield becomes extended through the use of these inventive ideas. Several of the ideas have been shown to improve the film uniformity.
While the protective shields described in these patents provide improved film uniformity, additional improvements are desired. For example, the protective shields as described include inert gas lines that enter the shield plenum near the end of the shield and then run down the length of the shield to the other end. The CVD system configuration typically requires the inert gas line to run down toward the shield, bend and then run along its length, and then bend back upward and exit from the assembly. Typically, the gas line has a series of holes which are usually, but not necessarily, evenly spaced along the length of the shield between the bends of the tube. The holes are not positioned in the bend area. While this design has proven to be superior to earlier designs, the bends in the tube are nonzero in lengths, and the first hole is placed where the tube becomes straight again. This results in low gas flow at the ends of the shield, and increased undesirable buildup of material in this region.
As mentioned above, high temperatures in the CVD reaction chamber create thermal stresses in the perforated screen. Over time this causes the screen to buckle and/or break. Prior art designs have addressed this problem. For example, U.S. Pat. No. 5,849,088 describes a protective shield that provides for expansion and contraction of the perforated front wall. While this is a significant advance, the assembly is relatively complex and can be expensive to manufacture.
Accordingly, it is desirable to provide an improved protective gas shield apparatus which minimizes build-up of materials at the ends, and which permits expansion and contraction of the perforated front wall in a less costly assembly.
It is a object of the present invention to provide a protective gas shield apparatus for use during chemical vapor deposition processing, and in particular to provide a gas shield assembly for protecting the exposed surfaces of a gas injector.
It is a further object of the present invention to provide a gas shield assembly which provides substantially uniform inert gas flow along the length of the shield.
It is a further object of the present invention to provide a simplified, less costly gas shield assembly which permits expansion and contraction of the perforated front wall.
The foregoing and other objects of the invention are achieved by a protective gas shield comprising a shield body having a back wall with flanges extending from the sides and ends of said body with a perforated sheet supported spaced from said back wall by said flanges whereby the back wall and perforated sheet form a plenum. At least one conduit is spaced inwardly from the sides of said shield body extending into said plenum for supplying an inert gas to said plenum whereby the gas flows outwardly through said perforated sheet. A retainer engages each end of said body and the edge of said perforated sheet to hold the perforated sheet on the shield body, while permitting expansion and contraction of the perforated sheet.
It is another object of the present invention to provide a shield assembly with removable and replaceable perforated front walls.
It is still another object of the present invention to provide a shield assembly which has prolonged life and which can be efficiently and economically manufactured and maintained.